Did you check <project dir="">\deployment\data\tmp\dojo\build-report.txt ?
Maybe there is another error. The check widget tool also generates this file.
But did you try from the modeler Tools / Check Widgets? That will at least give you the problamatic widget.
Regards,
Ronald
The deprecation-checker we build just runs through the widgets and checks for any patterns that might contain a deprecation which will break in Mendix 6. So it doesn't tell you anything about outdated things actually.
I'll check what this error means. It might be due to a non AMD-structure of a widget, or something like that. Like Marcel said, if you open that build report the next time, it will tell you where it fails (usually it has to do with dependencies not being found).